There is no extra money to cover industrial action by resident doctors, NHS trust leaders have warned.
It comes amid “worrying demands” for “excessive rates” to provide cover for striking medics, NHS Providers said.
The British Medical Association (BMA) announced resident doctors in England would walk out for five days from 7am on July 25 amid a row over pay.
A survey of trust leaders carried out by NHS Providers received 88 responses from 70 NHS trusts in England.
Almost nine in 10 (89%) said strikes will have a negative impact on their trust’s ability to deliver their financial plan for the year.
The same proportion warned that walkouts will have a negative impact on waiting lists, while 83% said strikes will impact the delivery of patient care.
Daniel Elkeles, chief executive of NHS Providers, said: “Trust leaders respect the right to strike and want to work with BMA colleagues to minimise the harm to patients.
“That means ensuring adequate cover, which is very expensive, as we know from previous strikes.
“Providers are telling us loud and clear there is no extra money to cover this, and the unexpected cost is bound to impact on the services they can provide. Patients will suffer.
“It is especially worrying to see demands for excessive rates to provide this cover.”
According to NHS Providers, some NHS regions have set rates for medics doing additional work to be paid during strikes.
However, it warned the BMA’s rate card – a document that outlines minimum hourly rates for doctors working extra or locum shifts should be paid – shows much higher hourly payments.
“These rates are simply unaffordable and would mean cuts to services,” Mr Elkeles said.
“The withdrawal of labour by one staff group should not be seen as a financial opportunity for another.
“That would be outrageous in a situation where there can be no winners.”
The survey results come ahead of crunch talks between the BMA and the Government this week.
On Monday, Health Secretary Wes Streeting told MPs that the union is telling members not to inform trusts or their employers if they are striking, which Mr Streeting described as “unconscionable”.
He also told the Health and Social Care Committee that he “cannot fathom” how “any doctor in good conscience would make it harder for managers to make sure we have safe staffing levels”.
The BMA has said that resident doctors need a pay uplift of 29.2% to reverse “pay erosion” since 2008/09.
It comes after its members voted to accept a Government pay deal worth 22.3% on average over two years last September.
The 2025/26 pay deal saw resident doctors given a 4% uplift plus £750 “on a consolidated basis” – working out as an average pay rise of 5.4%.